Insulated cable joint

ABSTRACT

The conductor at a cable joint is insulated by successive layers of a rubber type tape, such as butyl rubber or ethylene propylene rubber tape, around the conductors and incorporating a plurality of layers of another type of tape, such as cross-linked polyethylene or polypropylene tape, which has a lower electric loss value than the rubber type tape, in successive single layers with at least two layers of the rubber type tape under each layer of the other type. The layers of rubber type tape are bonded to each other and each layer of the other type is preferably arranged in an open spiral so that the layer of rubber type tape above bonds to the one below. Also, each layer of the other type is preferably shrunk on the layer of the rubber type below it.

O United States Patent [1113,567,845

[72] Inventor George Bahder [56] References Cited Edison, J- UNITEDSTATES PATENTS 25:;- $2 52 1970 3,033,727 5/1962 Cram et al. 156/56 2 lPatented Mar- 2,1971 3 2?58 5 2 6/ 966 Bartos et al 174/84 [73] AssigneeGeneral Cable Corporation Primary Examme y New York, N.Y.AttorneySandoe, l-lopgood & Calimafde ABSTRACT: The conductor at a cablejoint is insulated by successive layers of a rubber type tape, such asbutyl rubber or I ethylene propylene rubber tape, around the conductorsand incorporating a plurality of layers of another type of tape, such ascross-linked polyethylene or polypropylene tape, which has [54] ig c i za n e gv a lower electric loss value than the rubber type tape, insuccesalms sive single layers with at least two layers of the rubbertype [52] US. Cl 174/84, tape under each layer of the other type. Thelayers of rubber 156/49, 15 6/56, 174/120 type tape are bonded to eachother and each layer of the other [51] Int. Cl H0lb 7/02, type ispreferably arranged in an open spiral so that the layer HOZg 15/08 ofrubber type tape above bonds to the one below. Also, each [50] Field ofSearch 174/84, 88, layer of the other type is preferably shrunk on thelayer of the rubber type below it.

I NVENTOR. GEORGE BAI'IDER BY \Nvvk KTTORNE Y8 PAIENIEDHAR 2197i SHEET 2[IF 2 WITH LOW LOSS CROSS LINKED POLYETHYLENE TAPE.

FIG?) IIII WITHOUT Low LOSS TAPE zbo 360 460 560 600160 INSULATIONTHICKNESS IN MILS INSULATED CABLE JOINT The present invention relates tocable joints where in the cable insulation is stripped from the endportion of the conductors which are joined. In particular the inventionrelates to insulation for such joints and to a method of insulatingthem.

This invention is particularly adapted for insulating the joints ofcable, such as extruded cable wherein the cable core, consisting of aconductor with a semiconductive shield around it, is encased ininsulation, has a semiconductive shield around it and is enclosed by asheath of metal, such as lead. At present joints in this type of cableare insulated either by molding insulation around them or by wrappingthem with dielectric insulating tape. Suitable insulation for the jointsshould avoid corona discharges at the operating voltage of the cable andalso have a high level of breakdown voltage. In general, coronadischarge is controlled by eliminating voids in the insulation; thelevel of breakdown voltage is determined by the electric loss value ofthe insulation.

Molded insulation for such joints can, by present technology, be madewith a high impulse level. However, it provides a relatively low levelof corona discharge voltage due to present technological problemsconcerning elimination of voids in the joint insulation.

The wrapped tape insulation for the joints are normally formed by layersof a rubber-type tape, such as butyl or ethylene propylene rubber tapes,that are stretchable and selfbonding so that, if they are carefully andtightly wrapped, the occurence of voids is minimized and the coronadischarge voltage level is kept high. But these tapes have high electricloss values resulting in low levels of breakdown voltage. The breakdownis of the thermoelectrical type and the breakdown voltage isproportional approximately to the square root of the insulationthickness.

Butyl or ethylene propylene rubber tapes are useful in joints rated upto 35-40 kv, but they are marginal or unsuitable for the joints rated 69kv. This is due to the fact that at reasonable thicknesses of the jointinsulation-up to 3 times the thickness of the cabletl'1e impulse and ACbreakdown voltages are relatively low, and very often are below thedesired value of the basic impulse level.

It is a principal object of the present invention to provide a wrappedtape form of insulation for cable joints, and a method of forming theinsulation on the joint with which the corona discharge level is keptdown to an acceptable value and in which the level of breakdown voltageis higher than can be provided with known forms of molded or wrappedtape joint insulation.

In accordance with the present invention, the insulation for the jointis formed by a plurality of layers of a stretchable, selfbondingdielectric rubber-type tape, such as butyl or ethylene propylene rubbertape, wrapped around the conductor, over any semiconductive shieldaround the conductor and the joint, with a single layer of another typeof tape which has a lower electric loss value than the rubber-type tape,such as crosslinked polyethylene or polypropylene, between each fewlayers of the rubber-type tape.

A layer of the other type tape, which will hereinafter be referred to asthe low-loss tape for convenience, is wound over every second, third,fourth or more layers of the rubber-type tape so that in the totalthickness of the insulation a plurality of separate layers of thelow-loss tapeat very least two and preferably more-are provided.

The use of the stretchable, self-bonding rubber type is adapted, as inknown forms of wrapped tape insulation, to be wound around the conductorand over itself tightly without wrinkles or creases so as to minimizethe occurrence of voids and to minimize the level of corona discharge.The inclusion of layers of low-loss tape prevents the buildup of hotspots in the insulation so as to increase the breakdown voltage. Byincorporating a plurality of layers of the low-loss tape as singlelayers spaced among the layers of the rubber-type tape, each singlelayer of low-loss tape may be wound on without itself causing voids, andby being interspersed throughout the thickness of the insulation, theseveral layers of the low-loss tape are well placed for effectivelypreventing the buildup of hot spots.

The cable joint insulation, and the method of forming it, in accordancewith the invention will now be described in more detail with respect tothe accompanying drawings in which:

FIG. 1 is a cross-sectional view longitudinally through a typical cablejoint insulated in accordance with the present invention;

FIG. 2 is an enlarged cross sectionof a portion of the joint insulationappearing in FIG. 1, showing the layers of tape schematically;

FIG. 3, is an enlarged view of a section of the cable conductor at thejoint illustrating a preferred manner of winding the successive tapelayers thereon; and

FIG. 4 is a graph showing a comparison of the measured values of thebreakdown voltages for joint insulation incorporating low-loss tape inaccordance with this invention and for joint insulation from which thelow-loss tape was omitted.

Referring to FIGS. 1 and 2, typical cable, for the joints of whichinsulation in accordance with this invention is particularly adapted tobe used, is an extruded cable having a conductor 10 coated with a shield11 of a semiconductive material and encased in insulation 12. Theinsulation 12 is coated with a shield 13 of semiconductive material andis enclosed in a sheath 14 metal, such as lead.

As indicated in FIG. 1, for joining sections of the cable sheath 14, thesemiconductive shield 13, the insulation 12 and the semiconductiveshield 11 are removed from the end portions of the cables to bare theend portions of the conductors 10.'The insulation 12 is preferablytapered down to the bared end portions of the conductors. The ends ofthe respective conductors 10 are then brought together in a connector15, which is a short tube of copper or other conductive material. Theconnector 15 may be a split tube and is usually crimped onto theconductor ends.

A shield 16 of semiconductive material is placed around the connector 15and the adjacent bared portions of the conductors 10 and insulation 17,which consists of layers of a rubber type dielectric tape 18 with singlelayers of a low-loss tape 19 interspersed therein as illustrated in FIG.2, is wrapped around the joint area. The insulation 17, which thusreplaces the portion of cable insulation 12 removed for making thejoint, will customarily be much thicker than the normal cable insulation12, as shown. The nature of the insulation 17 and methods of applying itin accordance with the invention are subsequently described in detail.After the insulation 17 is in place it is covered with a shield 20 ofsemiconductive material, the ends of which overlap or are otherwisejoined to the end edges of the shield 13 of conductive material that isaround the cable insulation 12 under the metal cable sheath 14. Then ametal sheath 21 is placed around the joint area over the semiconductiveshield 20 and connected at its ends to the ends of the metal cablesheath 14 to completely seal the joint. The metal sheaths 21 and 14 aresuitably joined by butting and welding or soldering their adjacent edgestogether. Y

The rubber type dielectric tape 18 used in forming the insulation 17 isa stretchable, self-bonding tape which is suitably a butyl tape orethylene propylene rubber tape. The low-loss tape 19 is a tape ofdielectric material which has a lower electric loss value than therubber-type tape 18. The tape 19 may be stretchable, but need not be,and is preferably a tape consisting essentially of cross-linkedpolyethylene or polypropylene.

The tapes 18 and 19 are wound around the cable joint area, the tape 18being lapped, with at least two layers of the rubber-type tape 18 undereach layer of the low-loss tape 19. A plurality of layers of thelow-loss tape 19 (at least two and preferably more) are provided assingle layers between multiple layers of the rubber-type tape 18throughout the total thickness of the insulation 17 which is thusbuilt-up in layers.

The rubber-type tape 18 is drawn taut and stretched as it is woundaround the joint area so as .to go on tight to. assure a good bondbetween layers and to prevent the creation of voids.

As illustrated in FIG. 3, in accordance with the preferred methodsuccessive layers of the rubber-type tape. 18 are wound on in oppositedirections. Each layer of low-loss tape 19 is wound on in the samedirection as the layer of tape 18 below it; the next layer of therubber-type tape 18 wound over it is then wound in the same direction,the next successive layer of the rubber-type tape 18 being wound in theopposite direction. The low-loss tape 19 is preferably wound in shorterlays than the rubber-type tape 18 and is preferably wound in an openspiral so that thelayer of rubber-type tape 18 over a layer of thelow-loss tape bonds through the openings to the layer of rubber-typetape below.

In a preferred form using cross-linked polyethylene low-loss tape l9,the tape may be heated to a temperature of 130 F. as it is appliedso'that it shrinks when it cools, tightening it on the tape layers belowto further assure the absence of voids.

As previously discussed, the inclusion of separate layers of low-losstape 19 interspersed between layers of rubber-type tape 18 as describedprovides thermal barriers which prevent the buildup of hot spots andincreases the breakdown voltage level of the insulation 17. And, byeliminating the creation of voids as much as possible, the level ofcorona discharge is minimized.

FIG. 4 is a graph comparing the impulse voltage breakdown stress ininsulation incorporating layers of cross-linked polyethylene low-losstape 19 according to this invention with the impulse voltage breakdownstress in insulation from which the low-loss tape was omitted. Theinsulation specimens were at a temperature of 90 C., and the breakdownvoltage stresses are shown in volts per mil of insulation thickness. Asindicated by this graph, the inclusion of the low-loss tape inaccordance with this invention increases the stress level by more than200 volts per mil. in insulation 200 mils. thick; for insulation 2000mils. thick, the increase in stress level is over 150 volts per mil.

1 claim:

1. A method of insulating a cable joint wherein the cable insulation hasbeen removed from the adjacent endportions of two cable conductors whichare joined, said method comprising successively wrapping at least twolayers of a stretchable, self-bonding dielectric rubber-type tapespirally around the uninsulated portions of joined conductors, wrappinga layer of another type of dielectric tape, which has a lower electricloss value than said rubber-type tape, spirally around the outer layerof said rubber-type tape, and similarly wrapping additional layers ofsaid rubber-type tape and said other type tape to provide a plurality oflayers of said other type tape with at least two layers of saidrubber-type tape under each layer of said other type tape.

2. The method of claim 1 characterized by winding each successive layerof i said rubber-type tape in the opposite direction from the layerdirectly below, winding each layer of said other type of tape in thesame direction as the layer of said rubber-type tape directly below, andwinding each layer of said rubber type above a layer of said other typetape in the same direction as the latter layer of said other type tape.

3. The method of claim 1 characterized by winding the layers of saidother type of tape with shorter lays than the layers of said other type.I

4. The method of claim 1 in which said other type of tape is wound in anopen spiral.

5. The method of claim 1 in which said rubber type of tape is wound inoverlapping spiral turns.

6. The method of claim 1 in which said rubber-type tape is from thegroup consisting of butyl rubber tapes and ethylener propylene rubbertapes.

7. The method of claim 1 in which said other type of tape is from thegroup consisting of cross-linked polyethylene tape and ol ro lene ta e.

8. lh e me hod of c aim l in which said other type of tape iscross-linked polyethylene and is at a temperature of about 130 C. whenit is applied whereby it shrinks in situ when it cools.

9. A cable joint wherein insulation around the joined conductorscomprises a plurality of layers of a dielectric rubber type tape bondedto each other and a plurality of layers of another type of dielectrictape, which has a lower electric loss value than the rubber-type tape,interspersed between layers of the rubber-type tape with at least twolayers of the rubbertype tape between each two layers of the said othertype.

10. The cable insulation of claim 9 characterized by winding each layerof said other type tape in an open spiral, and wherein the layer ofrubber-type tape above a layer of the other type tape is bonded throughthe openings in said spiral to the layer of rubber-type tape below.

11. The cable joint ofclaim 9 in which said rubber-type tape is selectedfrom the group consisting of butyl rubber tapes and ethylene propylenerubber tapes.

12. The cable joint of claim 9 in which said other type of tape isselected from the group consisting of cross-linked polyethylene tape andpolypropylene tape.

1. A method of insulating a cable joint wherein the cable insulation hasbeen removed from the adjacent end portions of two cable conductorswhich are joined, said method comprising successively wrapping At leasttwo layers of a stretchable, selfbonding dielectric rubber-type tapespirally around the uninsulated portions of joined conductors, wrappinga layer of another type of dielectric tape, which has a lower electricloss value than said rubber-type tape, spirally around the outer layerof said rubber-type tape, and similarly wrapping additional layers ofsaid rubber-type tape and said other type tape to provide a plurality oflayers of said other type tape with at least two layers of saidrubber-type tape under each layer of said other type tape.
 2. The methodof claim 1 characterized by winding each successive layer of saidrubber-type tape in the opposite direction from the layer directlybelow, winding each layer of said other type of tape in the samedirection as the layer of said rubber-type tape directly below, andwinding each layer of said rubber type above a layer of said other typetape in the same direction as the latter layer of said other type tape.3. The method of claim 1 characterized by winding the layers of saidother type of tape with shorter lays than the layers of said other type.4. The method of claim 1 in which said other type of tape is wound in anopen spiral.
 5. The method of claim 1 in which said rubber type of tapeis wound in overlapping spiral turns.
 6. The method of claim 1 in whichsaid rubber-type tape is from the group consisting of butyl rubber tapesand ethylener propylene rubber tapes.
 7. The method of claim 1 in whichsaid other type of tape is from the group consisting of cross-linkedpolyethylene tape and polypropylene tape.
 8. The method of claim 1 inwhich said other type of tape is cross-linked polyethylene and is at atemperature of about 130* C. when it is applied whereby it shrinks insitu when it cools.
 9. A cable joint wherein insulation around thejoined conductors comprises a plurality of layers of a dielectric rubbertype tape bonded to each other and a plurality of layers of another typeof dielectric tape, which has a lower electric loss value than therubber-type tape, interspersed between layers of the rubber-type tapewith at least two layers of the rubber-type tape between each two layersof the said other type.
 10. The cable insulation of claim 9characterized by winding each layer of said other type tape in an openspiral, and wherein the layer of rubber-type tape above a layer of theother type tape is bonded through the openings in said spiral to thelayer of rubber-type tape below.
 11. The cable joint of claim 9 in whichsaid rubber-type tape is selected from the group consisting of butylrubber tapes and ethylene propylene rubber tapes.
 12. The cable joint ofclaim 9 in which said other type of tape is selected from the groupconsisting of cross-linked polyethylene tape and polypropylene tape.